Project Summary/Abstract The development of multicellular organisms is driven by the activity of stem cell populations. Stem cells reside in microenvironments or ?niches? that carefully balance the processes of cell self-renewal and differentiation. Disruptions in this balance can lead to severe developmental abnormalities, including congenital defects and cancer. Uncovering the processes regulating stem cell behavior will lead to more effective treatments and cures for such conditions. Our long-term objectives include identifying and characterizing the factors involved in maintaining the stem cell niche and controlling stem cell proliferation. To achieve these goals, we will take advantage of the easily accessible stem cell populations of the model plant Arabidopsis thaliana. Our first specific aim is to characterize key transcriptional regulators that maintain stem cell function by repressing differentiation-promoting genes. We have identified the Arabidopsis histone deacetylase HDA19, corepressor TPL, and transcription factor FD as key repressors of fate-specifying genes in multiple stem cell niches. These include the reproductive shoot apical meristem, responsible for reiteratively initiating flowers, and the floral meristem, which produces individual floral organs. We will identify and functionally characterize protein interactors that form complexes with these three transcriptional regulators to control gene expression in meristems, thereby revealing novel mechanisms underlying stem cell maintenance. In our second aim, we will investigate how reproductive output and age promote the arrest of stem cell activity. Using multiple transcriptomic and high-throughput protein interaction approaches, we have generated a promising list of transcription factors that control stem cell quiescence in response to seed production or age. Notably, related subfamilies of transcription factors appear to modulate meristem function in response to both these developmental variables. We will determine the precise roles of these factors in the promotion of stem cell quiescence through genetic, molecular and biochemical analyses. Moreover, by manipulating the expression of these quiescence-associated genes, we expect to controllably alter the duration of the plant reproductive stage. Collectively, our approaches will characterize the protein factors and molecular mechanisms that modulate stem cell proliferation and niche maintenance in multicellular eukaryotes.